Effective Stress Levels Research Articles

Experimental investigation of gas seepage characteristics in coal seams under gas-water-stress function

Seepage experiences were conducted on coal samples with diverse levels of moisture content, gas pressure, and effective stress to investigate how gas seepage in a coal seam is affected by the interaction of gas, water, and stress. The results of the study revealed the intricate relationship between these factors and their impact on the permeability and seepage behavior of coal. The findings indicate that, with increasing gas pressure, the permeability of coal specimens containing different levels of moisture varies distinctly. When coal samples have low moisture content, their permeability displays a pattern of “increase - decrease - increase” as gas pressure increases. However, with the further increase of moisture content, the “increase-decrease” trend of permeability with the increase of gas pressure disappears. The relationship between permeability and effective stress can be modeled using either a quadratic or logarithmic function. On the other hand, the connection between permeability and moisture content, can be represented by a quadratic or exponential function. At low levels of moisture content, gas pressure has the most pronounced effect on permeability of coal samples, followed by moisture content and effective stress. Conversely, at high levels of moisture content, the most influential factor is moisture content, followed by gas pressure and effective stress. Finally, a model of permeability has been developed that takes into account the collective impacts of gas pressure, moisture content, and effective stress. The research outcomes can establish a basis for optimizing gas recovery from coal seams.

Evidence for Low Effective Stress Within the Crust of the Subducted Gorda Plate from the 2022 December Mw 6.4 Ferndale Earthquake Sequence

Abstract Stress levels on and adjacent to megathrust faults at seismogenic depths remain a key but difficult-to-constrain parameter for assessing seismic hazard in subduction zones. Although strong ground motions have been observed to be generated from distinct, high-stress regions on the down-dip end of the megathrust rupture areas in many great earthquakes, we lack direct constraints on the stress level in the lower seismogenic portion of the Cascadia megathrust. On 20 December 2022, an Mw 6.4 strike-slip earthquake occurred near Ferndale, California, in southern Cascadia and likely ruptured the Gorda slab crust in the lower seismogenic portion, providing an opportunity to assess the stress level in this region. Here, we relocate the Ferndale mainshock and the first two weeks of aftershocks using a high-resolution 3D velocity model and estimate rupture dimensions, directivity, and stress drop for several Mw 4–5 aftershocks and recent earthquakes. The aftershocks define a strike-slip fault in the slab crust striking east-northeast, consistent with the mainshock focal mechanism. The orientation of this fault is about 45° off the ideally oriented fault plane given the stress state in the slab. The aftershock zone is extensive and broad in the forward direction of the mainshock rupture but still constrained within the volume of high VP/VS in the slab crust. Our stress-drop estimates are generally lower for Mw 4–5 earthquakes located in the slab crust compared to those a few kilometers deeper in the slab mantle. Combined, our results support a relatively low effective stress level in the vicinity of the megathrust in the lower portion of the seismogenic zone in southern Cascadia, likely due to elevated fluid pressures. Consequently, the ground motion in the onshore region above this low-stress seismogenic portion in southern Cascadia may not be as intense as that observed during great earthquakes in other subduction zones.

In-plane characteristics of a multi-arc re-entrant auxetic honeycomb with enhanced negative Poisson's ratio effect and energy absorption

Based on conventional re-entrant auxetic honeycomb (CRAH), a multi-arc re-entrant auxetic honeycomb (MARAH) is proposed. The mechanical properties of the honeycomb can be significantly improved by introducing multiple arcs. Through theoretical analysis, finite element analysis, and experiment, the influence of arc radius, arc angle, and cell wall thickness on effective Poisson's ratio, effective Young's modulus, energy absorption, and stress level are investigated. There are significant differences between MARAH and CRAH in Poisson's ratio, deformation mode, stress level, and energy absorption. Compared with CRAH, MARAH has a better negative Poisson's ratio effect, wider Poisson's ratio adjustable range, better energy absorption, and superior stability. In addition, the reduction of the yield stress can effectively reduce the damage of impact load on the honeycomb. The research results can provide new ideas for the design and application of new metamaterials.

Assessing swelling-induced damage in shale samples during triaxial testing

In shale testing, understanding the impact of effective stress and saturation conditions is crucial for accurate material behaviour assessment and parameter determination. In some cases, saturation in triaxial testing starts at low effective stress before ramping up for shearing. However, when in contact with water (or saline water), shales are prone to swelling, particularly at low effective stress levels, which can induce fissures and alter material properties. This study investigates the influence of fluid saturation strategies and stress/pressure variations on the mechanical behaviour of shales, particularly under low effective confinement. Building upon the comprehensive testing campaign (>140 tests) in Crisci et al. (2024), additional tests were conducted on Opalinus Clay shale, focusing on sample saturation methods and loading histories before shearing. The conditions under which tested specimens experience damage were detected through diagnostic indicators such as differences in stress path and lower strength and stiffness compared to intact specimens with identical basic properties. Micro CT scanning confirms that damage is related to the development of fissures. The volumetric changes in specimens were quantified throughout the testing phases and thresholds for tolerable strains and effective stresses, specific to this material, were established. Comparative analysis with Opalinus Clay from shallower depths and other shales globally revealed consistent findings. Notably, it is shown that, for all shale types analyzed, a linear failure envelope emerges in the low to intermediate effective stress regime when filtering out "damaged" specimens. This suggests that non-linear failure envelopes observed in some cases may stem from exposing specimens to low effective stress before shearing.

Data-enhanced design charts for efficient reliability-based design of geotechnical systems

This paper proposes a new design chart approach for reliability assessment, which enables clear visualization of the representative soil shear strength parameters under various reliability levels and effective stress levels. Utilizing the design charts, reliability assessment or reliability-based design can be performed with significantly reduced numbers of evaluations of the geotechnical system response. The design charts are established solely based on the probability distributions of soil parameters, and are applicable to a variety of geotechnical problems involving the same soil type. For practical illustration of the proposed approach, design charts are produced from the shear strength databases of saprolitic soils and colluvial soils in Hong Kong, and then applied to the reliability-based design of a slope with soil nail reinforcements. The ensuing design solutions require much fewer soil nails compared to the conventional design practice, while also achieving a better system reliability. The same charts are then applied to the reliability-based design of a retaining wall, where a series of design options are identified with equivalent reliability index against overturning failure and pullout failure. Through the proposed approach, the use of design charts promotes efficient reliability-based design of geotechnical systems with rational incorporation of reliability concepts.

TECTONOPHYSICAL ZONING OF ACTIVE FAULTS OF THE BAIKAL RIFT SYSTEM

Tectonophysical zoning of active faults of the Baikal rift system (BRS) was performed based on the degree of hazard caused by the generation of earthquakes of magnitude 7.0 and higher. The first basis for this procedure was the results of the natural stress state reconstruction, earlier performed from seismological indicators of rupture deformations (earthquake focal mechanisms). The second important element of fault zoning was electronic maps of active faults of Eurasia hosted on the GIN RAS sever. Within the algorithmic framework for Rebetsky’s method of cataclastic analysis, both of these datasets allowed calculating the Coulomb stresses for the segments of the BRS faults. During the study, a development of the cataclastic method has been carried out in using a diagram of brittle fracture as the Mohr–Coulomb model, considering a decrease in the range of positive Coulomb stress values with an increase in effective normal stress levels. Such an approach provides a more reliable identification of the fault segments with the maximum Coulomb stress levels. The performed calculations showed that in the BRS crust there are several up to 50 km long fault segments having critically high (80–100 % of the maximum) and high (40–80 % of the maximum) Coulomb stress levels. It is these corebased hazardous zones that are considered as places where seismogenic ruptures of future М>7.0 earthquakes may start. There have been distinguished three zones that present such hazard: 1) in the western segment of the BRS in the western part of the Tunka Valley in the Tunka, Khamardaban-Mondy and Baikal-Mondy fault systems; 2) in the Selenga River delta in the Proval, Delta, Ust-Selenga and Sakhalin-Enkhauk fault systems; 3) within the northeastern flank of the BRS on the fault system of the Muyakan basin (along the North Muya ridge). It is proposed to perform tectonophysical monitoring of changes in the stress state of these three zones and make observations of their surface motions using remote sensing methods.

A hypoplastic model for pre- and post-liquefaction analysis of sands

This article proposes an extended constitutive model for liquefaction analysis of sands. The proposed model is formulated based on the hypoplastic model for granular soils by von Wolffersdorf (1996), enhanced with Intergranular Strain Anisotropy by Fuentes et al. (2019), which is usually referred to as ISA-hypoplasticity. The proposed extended model includes some modifications that are indispensable for the correct prediction of liquefaction-related analysis: (i) a new Lode-angle-dependent function for post-liquefaction shear strain accumulation, (ii) a modification for fabric change effects, (iii) the theory of the so-called semifluidized state. The first modification allows the model to predict shear strain accumulation in extension and compression during cyclic mobility. The second and third modifications, were initially developed by Liao et al. (2022) for hypoplasticity with conventional intergranular strain based on the pioneer work by Barrero et al. (2020) for Sanisand, and enable the model to reproduce fabric change effects upon loading reversal, and stiffness and dilatancy degradation at low effective stress levels, respectively. With the consideration of the new Lode-angle function, the proposed model realistically predict the increasing shear strain accumulation in both extension and compression without adopting a circular critical state surface, as adopted in Liao et al. (2022). The proposed model was carefully calibrated and validated based on a series of monotonic and cyclic triaxial tests on Zbraslav and Karlsruhe fine sands, considering various testing conditions. The comparison between experimental measurements and numerical predictions of undrained cyclic tests suggests that the proposed model accurately describes the pre- and post-liquefaction stages, as well as the stress attractors.

Influence of stress history on thermal conductivity of saturated fine-grained soils

This research investigated variations in thermal conductivity k for normally consolidated (NC) and overconsolidated (OC) saturated fine-grained soils at different levels of vertical effective stress. A series of undrained heating tests were performed using a specially designed consolidometer with central radial heating arrangement and temperature measurements within a soil specimen. For different soils investigated in this research, k correlated well with soil consolidation characteristics (compression index λ and swelling index κ) and vertical effective stress σvʹ at both NC and OC states. However, the rates of increment of steady- and transient-state values of k ( kst and ktr, respectively) with σv′ differed from one soil to another. For OC clays, both ktr and kst increased with preconsolidation stress σc′. A set of equations were proposed to estimate both kst and ktr for saturated fine-grained soils as functions of σv′, λ, κ, and stress history. At a particular value of σvʹ, the dependence of k on overconsolidation ratio (OCR) and σc′ marked the importance of considering stress history in the determination of k. The influence of soil mineral composition on k was also explored. Quantification of soil mineralogical composition and available data on mineral thermal conductivity enabled prediction of ktr with reasonable accuracy.

Rupture Dynamics of Cascading Earthquakes in a Multiscale Fracture Network

AbstractFault‐damage zones comprise multiscale fracture networks that may slip dynamically and interact with the main fault during earthquake rupture. Using 3D dynamic rupture simulations and scale‐dependent fracture energy, we examine dynamic interactions of more than 800 intersecting multiscale fractures surrounding a listric fault, emulating a major listric fault and its damage zone. We investigate 10 distinct orientations of maximum horizontal stress, probing the conditions necessary for sustained slip within the fracture network or activating the main fault. Additionally, we assess the feasibility of nucleating dynamic rupture earthquake cascades from a distant fracture and investigate the sensitivity of fracture network cascading rupture to the effective normal stress level. We model either pure cascades or main fault rupture with limited off‐fault slip. We find that cascading ruptures within the fracture network are dynamically feasible under certain conditions, including: (a) the fracture energy scales with fracture and fault size, (b) favorable relative pre‐stress of fractures within the ambient stress field, and (c) close proximity of fractures. We find that cascading rupture within the fracture network discourages rupture on the main fault. Our simulations suggest that fractures with favorable relative pre‐stress, embedded within a fault damage zone, may lead to cascading earthquake rupture that shadows main fault slip. We find that such off‐fault events may reach moment magnitudes up to Mw ≈ 5.5, comparable to magnitudes that can be otherwise hosted by the main fault. Our findings offer insights into physical processes governing cascading earthquake dynamic rupture within multiscale fracture networks.

Effects of Prestressing Magnitude and Position on Seismic Performance of Unbonded Prestressed Concrete Beams

To study the effects of the jacking stress level, height and strength ratio of the prestress tendons (λ) on the seismic performance of unbonded prestressed concrete (UPC) beams, six UPC beams and one reinforced concrete (RC) beam were tested under cyclic loads. The hysteretic characteristics, skeleton curves, ductility properties, energy dissipation capacity, strain distribution of reinforcement and self-centering capability of the specimens were studied and discussed. Numerical parameter analysis was also carried out by using OpenSees. The results indicate that three failure modes of UPC beams under cyclic loading were observed, namely the tension-failure mode involving a broken rebar, the compression-failure mode involving concrete crushing and the balanced failure. By considering the influence of the prestress position and magnitude, the modified reinforcing index ω was proposed to determine the failure mode. The ω is suggested to be less than 0.3 to ensure sufficient ductility. The effective stress level is linearly and positively related to the stiffness from cracking to yield Kcr and the ultimate bearing capacity of the UPC beam under cyclic loading. The stiffness of the UPC beam is slightly larger than that of the RC beam before yielding, and significantly greater than that of the RC beam after yielding. Due to the large strength reserve after yielding, the integrated seismic performance of the UPC beam is similar to that of the RC beam. When the λ was unchanged, the increase in the relative height of the prestressed tendons αh is beneficial for the overall performance factor F, ductility and crack control. The stiffness degradation performance depends on the λ but is independent of the αh. The total energy dissipation of the non-tensioned UPC specimen was 59% higher than that of the RC beam. The cumulative total energy dissipation of the tensioned UPC specimen was only 13% lower than that of the RC beam with the same number of cycles, indicating that the UPC specimen had a considerable energy dissipation capacity.

Laboratory investigation of the cyclic loading behaviour of intact and de-structured chalk

Chalk is a soft biomicrite composed of silt-sized crushable CaCO3 aggregates. Chalk’s response to cyclic loading depends critically on its sensitive micro fabric and state, which may be altered significantly by high-pressure compression, dynamic impact or prior large-strain repetitive shearing. This paper reports high-resolution undrained cyclic triaxial experiments on low- to medium-density intact chalk and chalk de-structured by dynamic compaction to model the effects of percussive pile driving. The intact chalk manifested stable and nearly linear visco-elastic response under a wide range of the one-way, stress-controlled cyclic loading conditions imposed. However, high level cycling led to sudden failures that resembled the fatigue response of metals, concretes and rocks, with little sign of cyclic damage before sharp pore pressure reductions, non-uniform displacements and finally brittle collapses. However, the de-structured chalk’s response to high-level undrained cycling resembles that of silts, developing both contractive and dilative phases that led to pore pressure build-up, leftward effective stress-path drift, permanent strain accumulation, cyclic stiffness losses and increasing damping ratios. Results from exemplar tests are presented to illustrate these key features and demonstrate how chalk’s undrained cyclic shearing characteristics depend also on effective stress level. The experimental outcomes provide significant scope for developing constitutive and empirical relationships or predictive tools to enable the interpretation and design of driven pile foundations in chalk and other chalk-structure interaction related problems under cyclic loading.

Investigation of thermal effects on the saturated shear behaviour of a clayey sand-structure interface

The mechanical behaviour of soil-structure interfaces at various temperatures plays a key role in predicting the performance of energy piles, such as their ultimate bearing capacity and settlement under heating and cooling. The experimental data was limited in the literature, and previous studies used clay and clean sand. In this study, a modified direct shear apparatus that can control temperature was developed. To control interface temperature, a refrigerated/heated circulating bath is connected to channels in the lower shear box and then heated/cooled water is circulated. The interface can be heated/cooled through heat exchange with circulating water. Three series of tests were conducted at various temperatures of 8, 20 and 42 °C and effective normal stress levels of 50, 150 and 300 kPa. The soil specimen was recompacted clayey sand with a 95% degree of compaction. The results indicate that the shear strength of saturated soilstructure interfaces decreases with increasing temperature. This is likely because temperature elevation results in a reduction of interface roughness and a partial increment of void ratio in the shear zone.

The monotonic behaviour of a low- to medium-density chalk through in-situ and laboratory characterisation

Chalk is a highly variable cemented biomicrite limestone that can show widely different rock strengths and patterns of micro to macro fissuring and jointing, due to variations in depositional environments and local geological histories. This paper describes the characterisation of a very weak to weak, low- to medium-density chalk through in situ profiling and laboratory testing, which provided new insights into the geomaterial’s mechanical behaviour. The chalk de-structures when taken to large strains, leading to remarkably high pore pressures beneath penetrating cones and degraded responses in full-displacement pressuremeter tests. Laboratory tests on carefully formed specimens explored the chalk’s unstable structure and marked time-, rate- and pressure dependency. A clear hierarchy was found between profiles of peak strength with depth from Brazilian tension, drained and undrained triaxial and direct simple shear tests conducted from in-situ stress conditions. Highly instrumented triaxial tests sheared from low confining stresses indicated stiffness anisotropy and showed very brittle failure behaviour from small strains. Progressively more ductile behaviour was seen as confining pressures were raised, with failures being delayed until increasingly large strains and finally stable critical states were attained. The chalk’s mainly sub-vertical jointing and micro-fissuring led to properties depending on specimen scale, with high-quality laboratory stiffness measurements significantly exceeding those obtained from in-situ geophysical testing, which far exceed the operational stiffnesses of the chalk mass. While compressive strength and stiffness appear relatively insensitive to effective stress levels, consolidation to higher pressures closes micro-fissures and reduces anisotropy. The results provided the basis for numerical analysis with advanced constitutive models that inform the interpretation of axial and lateral tests on driven piles and inform the development of new practical design methods.

Effects of exposure in seawater sea-sand concrete pore solution on fatigue performance of carbon FRP bars

The study aims to propose a methodology to predict the fatigue performance of carbon fiber-reinforced polymer (CFRP) bars under the seawater sea-sand concrete (SWSSC) environment. The effects of exposure in SWSSC pore solution on the fatigue damage mechanisms and fatigue performance of CFRP bars are presented. Firstly, the tensile fatigue tests of reference CFRP bars were performed under various stress levels with a constant stress ratio of 0.1. Secondly, the tensile fatigue tests of exposed CFRP bars subject to the simulated SWSSC pore solution were carried out. The tensile failure modes and fatigue damage mechanisms corresponding to different stress levels, were identified using scanning electron microscopy (SEM). Besides, the relationship between fatigue life expectancy and underlying fatigue damage mechanisms was carefully analyzed. Finally, the effective stress level method (ESLM) was proposed to evaluate the residual fatigue life of exposed CFRP bars based on the benchmark S–N curve. Combined with the degradation models (chemical etching/diffusion models) proposed in our previous research, ESLM could bridge corrosion fatigue life prediction of the exposed CFRP bars and the benchmark fatigue data of the non-exposed CFRP reinforcement. The predicted results have good correlations with the experimental data. This method can also provide valuable insight for the corrosion fatigue life prediction of other types of FRP composites.

Non-Destructive Methods and Numerical Analysis Used for Monitoring and Analysis of Fibre Concrete Deformations.

The aim of the research was to check the possibility of using the non-destructive method of acoustic emission to assess the condition of concrete without dispersed reinforcement and with various additions of curved steel fibres, during three-point bending. An important aspect of the research proposed in the article is the use of a hybrid method of analysis, which involves complementing the results of strength tests, the results of numerical calculations and the results of strain distributions recorded with a digital image correlation system (DIC System, in this research GOM Suite optical system). The operation of the concrete material under load, depending on the amount of fibres added, is reflected in the recorded acoustic emission (AE) signals. The differences concern the number of signals of individual classes and their distribution over time. The differences exist for both low and high load values, which confirms the possibility of using the acoustic emission method to monitor the condition of the material. It was shown that the numerically determined effective stress levels decreased as the proportion of steel fibres in the concrete increased, while the maximum levels of the first principal stresses increased. During the analyses, a preliminary comparison of the deformation results obtained using the finite element method and the DIC System was also carried out.

Effects of time-dependent deformation of shale on the integrity of a geological nuclear waste repository

Safety assessment of geological nuclear waste repositories is essential for the permanent disposal of spent nuclear fuels and high-level radioactive waste. The long-term integrity of the host rock as well as the engineered barrier system (e.g., bentonite buffer) surrounding nuclear waste canisters is of particular importance as decay heat from nuclear waste canisters, which will last over thousands of years, significantly disturbs the thermo-hydromechanical (THM) state of the repository. In this study, THM coupled simulations were carried out to investigate the effect of time-dependent deformation (i.e., creep) of shale on the long-term integrity of a generic subsurface nuclear waste repository. The Norton-Bailey creep model, which is also known as the Lemaitre-Menzel-Schreiner model, was employed to simulate the power-law type creep that is observed in shales. The TOUGH-FLAC simulator was employed for the THM coupled modeling of the repository. The objective of this study is to assess the effect of creep in different shales (i.e., high creep shale vs. low creep shale) on long-term stress and permeability changes in the repository. Results show potential advantages of constructing repositories in the high creep shale, as deviatoric stress levels in the formation decreased to zero in 100 years since the emplacement of nuclear waste canisters and the permeability also decreased to the undamaged, intrinsic levels in 10,000 years. Also, mean effective stress levels in the bentonite buffer increased by 100% in the high creep shale case relative to the low creep shale case at 10,000 years due to creep-induced contraction of the nuclear waste disposal tunnel. However, in earlier periods (e.g., 1000 years), the stress levels in the bentonite buffer were twice smaller in the high creep shale case than in the low creep shale case, which shows a tradeoff between the intermediate- (∼1000 years) and long-term (>10,000 years) compaction levels in the bentonite buffer depending on the creep characteristics of the host shale.

Evaluating CO2 breakthrough in a shaly caprock material: a multi-scale experimental approach

The potential of underground CO2 storage relies on the sealing efficiency of an overlaying caprock that acts as a geological barrier. Shales are considered as potential caprock formations thanks to their favourable hydro-mechanical properties. In this work the sealing capacity of Opalinus Clay shale to CO2 injection is studied by means of capillary entry-pressure and volumetric response. The overall objective of this work is to contribute to the safe design of a CO2 injection strategy by providing a better understanding of the geomechanical response of the caprock material to CO2 injection and eventual breakthrough at different scales. This is achieved by relating lab-measured hydro-mechanical properties of the studying caprock material (porosity, permeability, volumetric response) to field-related parameters (effective stress, injection pressure). A number of CO2 breakthrough tests is performed in Opalinus Clay samples under two different scales, meso and micro. At the meso-scale, CO2 injection is performed in oedometric conditions under different levels of axial effective stress in both gaseous or liquid phase. In parallel, the material’s transport properties in terms of water permeability are assessed before CO2 injection at each corresponding level of effective stress. The impact of CO2 phase and open porosity on the material’s CO2 entry pressure are demonstrated. The correlation between measured entry pressure and absolute permeability is discussed. A second testing campaign at a smaller scale is presented where CO2 breakthrough is for the first time identified with in-situ X-ray tomography. CO2 injection is performed under isotropic conditions on an Opalinus Clay micro-sample (micro-scale), and CO2 breakthrough is identified through quantitative image analysis based on the measured localised volumetric response of the material. This innovative methodology provides important insight into the anisotropic response of this complex material that is indispensable for its representative modelling in the context of safe geological CO2 storage.

Dynamic enhancement mechanism of energy absorption of multi-cell thin-walled tube

In order to study dynamic response of multi-cell thin-wall tubes with small aspect ratio and diameter thickness ratio under high velocity impacts of lightweight devices, the mass block impact tests were performed on three types of thin-walled tubes including circular tube (CirT), multi-cell tube (MT) and polyurethane foam (PUR) filled multi-cell tube (FMT). The dynamic enhancement mechanism of collapse force of MTs was discussed based on numerical simulation and deformation pattern analysis of MTs and FMTs. The results indicated that the dynamic enhancement coefficient of mean crushing force (MCF) was 1.0–1.17 and 1.12–1.3 for MTs and FMTs, respectively. The dynamic enhancement of EA was 24–29.2% for CirTs, 0–14.7% for MTs and 9.42–21.16% for FMTs, compared to the quasi-static compression. The multi-cellularization tended to degenerate the sensitivity of collapse force of MTs to the loading rate, while the foam filling increased the rate sensitivity. With the impact velocity increasing from 30 to 70 m/s, the folding half-wavelength of MTs and FMTs decreased about 2–5 mm. Also, the effective stress level of the MTs increased with the impact velocity and the material strain hardening mechanism. The folding pattern of MTs could be improved by foam filling and enhancing the impact velocity.

Advanced in situ and laboratory characterisation of the ALPACA chalk research site

Low- to medium-density chalk at St Nicholas at Wade, UK, is characterised by intensive testing to inform the interpretation of axial and lateral tests on driven piles. The chalk destructures when taken to large strains, especially under dynamic loading, leading to remarkably high pore pressures beneath penetrating cone penetration testing and driven pile tips, weak putty annuli around their shafts and degraded responses in full-displacement pressure-meter tests. Laboratory tests on carefully formed specimens explore the chalk's unstable structure and markedly time- and rate-dependent mechanical behaviour. A clear hierarchy is found between profiles of peak strength with depth of Brazilian tension, drained and undrained triaxial and direct simple shear tests conducted from in situ stress conditions. Highly instrumented triaxial tests reveal the chalk's unusual effective stress paths, markedly brittle failure behaviour from small strains and the effects of consolidating to higher than in situ stresses. The chalk's mainly sub-vertical jointing and micro-fissuring lead to properties depending on specimen scale, with in situ mass stiffnesses falling significantly below high-quality laboratory measurements and vertical Young's moduli exceeding horizontal stiffnesses. While compressive strength and stiffness appear relatively insensitive to effective stress levels, consolidation to higher pressures closes micro-fissures, increases stiffness and reduces anisotropy.

Influence of compaction on small-strain shear modulus of iron ore tailings

Understanding the geotechnical properties of iron ore tailings is one of the biggest challenges currently faced by the mining industry. The brittle behaviour of these tailings has brought the importance of small-strain stiffness to the geotechnical forefront. However, lack of knowledge and information about the behaviour of iron ore tailings still exists. The results and analysis of a laboratory programme aimed at assessing the small-strain stiffness of tailings materials are presented in this paper. The materials tested were produced during the iron ore treatment process. Bender elements were used to measure shear wave velocities and evaluate dynamic shear moduli at different effective stress levels resulting from isotropic consolidation tests. Three types of iron ore tailings with different grain size distributions were used – flotation tailings, slimes tailings and blended tailings. Reconstituted specimens were prepared at different densities (loose and dense conditions) to assess the effects of initial density (percentage compaction) on the shear modulus. The laboratory results were compared with empirical correlations for other soil types. The equations were ineffective at representing tailings materials containing large amounts of fines (slimes). The advantages and limitations of the equations are discussed and a new empirical equation that includes the degree of compaction is suggested.